With all your spare time how about coming up with a substitution chart plus
what changes are needed for a wide range of RF devices that are no longer in
production with some unobtainium? This would include VHF/UHF brick amps.
Carl
KM1H
----- Original Message -----
From: "Manfred Mornhinweg" <manfred@ludens.cl>
To: <amps@contesting.com>
Sent: Sunday, October 19, 2014 1:53 PM
Subject: Re: [Amps] Solid State Amps
Roger,
Small, generic LCD screens could be produced for pennies, or at most, a
few dollars as you can now purchase a 40" smart HD TV receiver for
$300-$400.
It's a matter of checking what a simple cellphone costs these days. Two
years ago I got my latest one, an old-style dumbphone in clamshell style.
That one has two displays, a monochromatic one outside and a pretty decent
color display inside. The whole phone cost like $30, including shipping
from China, and that's a price that wasn't subventioned by a phone
company! So the displays really must be cheap, when bought in the quantity
a cellphone make buys them.
> How many
transistors used in SS rigs over the last two decades are no longer
available?
That's not a problem, because essentially any transistor can be replaced
by a different one. But the problem does exist with specific ICs. A famous
(or rather, infamous) case is the DDS chips of the Kenwood TS-450, 690,
850, that seem to rot from the inside out due to some fabrication mistake.
Those ICs are specific, discontinued, and presently unavailable except
perhaps from someone who has a few stockpiled, and sells them at a very
stiff price. And there is nothing that can replace them in a plug-in
fashion. One would have to cook a new board with a different DDS chip and
a MCU or other circuit to translate the commands, to revive any of these
radios whose DDS chips went bad.
Mine went bad in time, when I still could get replacements without the
fabrication mistake, so I'm fine in this regard. But I know of several
hams who have dead radios of these models, and no way to fix them.
However, HRD and other Graphical interfaces can replace most of the front
panel, let alone the individual displays on today's rigs. If using HRD
you might not even notice a failed, non essential display.
It's true, but when you tie a radio to a computer, it's getting twice as
expensive, 5 times larger and heavier, and the power consumption is 10
times higher. So it's really good to be able to operate a radio without an
external computer!
And now, let's go to really interesting matters!
Warren,
** It seems to me that it's probably much easier to optimally implement
pre-distortion in a DDC/DUC (digital up-conversion / digital
down-conversion) software defined radio than in other radios. There are
simply very few places that errors can creep into the required
processing. The required precision and accuracy increase exponentially
as the desired reduction in IMD increases. So, having few error sources
and the benefit of double-precision floating point become important.
What does "double-precision" mean in this case? 32 bits, 64 bits, or how
much?
To have errors down 60dB, even 16 bit should be OK, for a single
operation. But I wonder how many math operations are involved in a radio's
whole processing. That surely calls for more bits of resolution in each
operation!
I saw that a DSP chip with integrated CODEC, aimed at mid level HiFi
applications, has a 56 bit processor! They strive to put any errors about
100dB down from the peak signal.
** 50V LDMOS amplifiers are probably the "most correctable" solid state
amps. Tube type amps also correct very well. 13.8V amps are more
difficult due to "memory effects."
What memory effects do you mean here? Dielectric absorption, thermal
effects, or what?
** No problem to correct an entire amplifier chain, at legal limit. No
extra hardware or software is required. One just feeds back a sample of
the output from the last stage to calculate the correction.
That's the nicest thing about SDRs. Once you have the interfaces to the
analog world up and running, and a good chunk of processing power, you can
do pretty much _anything_ at no additional cost.
** You cannot successfully correct by pre-distorting the MIC input to a
radio UNLESS you have VERY wide bandwidth from the MIC through the entire
transmitter. The IMD frequencies that you want to correct must be within
the bandpass of the correction path.
Here you caught me in a shaky position. It's where I touch the limits of
my present theoretical knowledge about the matter. If I set up an
amplifier that corrects amplitude nonlinearities, with a loop bandwidth of
30kHz, does this mean that any amplitude-error-caused IMD more than 30kHz
away from the center frequency will not be corrected at all? But that
could still be quite useful, given that IMD so far away from the center
frequency should anyway be pretty low.
At Ham Radio Friedrichshafen this past June, I gave a talk (approx. 30
min.) on the openHPSDR solution for pre-distortion. It's posted on the
openHPSDR web site. If any are interested, here's a link:
"*Warren Pratt,*NR0V, 2014.Digital Predistortion linearizes RF amplifiers
<http://video.openhpsdr.org/HRF2014/PureSignal1.2.mp4>(MP4)"
Is the contents of that video available in written form? Due to internet
connection limitations, I cannot download videos...
I hope this helps explain a little more about this technology and sparks
more interest in finding ways to reduce IMD!
It does! For several years I have been wanting to get seriously into DSP
and really do something myself in the area, but never have found the entry
door. Warren, can you perhaps suggest a practical, inexpensive way to set
up a kind of DSP breadboard, that would allow me to mount a processor,
suitable A/D, D/A sections, and then go along getting my feet wet
programming it? So far my only attempts at incorporating DSP into my own
projects has been by basic digital filtering functions in low frequency
applications. Mostly power line frequency. And implemented in integer
math, on 8-bit PICs!
Years ago I tried my hand with a DSP-93 kit that I got for free, but never
found out how to even start doing something useful with it.
Roger seems to be in the same boat I am.
Manfred
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